In general, a filler tube of a fuel tank includes a vent tube branched from the upper portion thereof and opened at a specified height in the fuel tank. In refueling, when the fuel level in the fuel tank reaches the position of the opening of the above vent tube, the vent tube is blocked by the fuel to close up the inside of the tank. Accordingly, the tank internal pressure is increased, and the fuel is raised in the filler tube by the above internal pressure. Then, when the fuel reaches the opening portion of the filler tube, the refueling is stopped by the actuation of an auto-stop of a fuel nozzle or the visual checking of this state by an operator, thus regulating the full-tank fuel level. At this time, the tank internal pressure is increased up to a pressure corresponding to the height of the fuel raised up to the opening portion of the filler tube.
Conventionally, there has known an internal combustion engine of a type that, to avoid an excessive rise or reduction in pressure inside a fuel tank caused by a variation in the outside air temperature and the like, in a vent passage for communicating the upper space of the fuel tank to a canister, there is provided a two-way valve for allowing a gas flow from a tank side to a canister side when the tank internal pressure is raised, and from the canister side to the tank side when the tank internal pressure is reduced. In such an internal combustion engine, the two-way valve is set in such a manner that the valve opening set-up pressure at the time of flowing the gas from the tank side to the canister side is equal to at least the tank internal pressure necessary for regulating the full-tank fuel level in feeding the fuel.
However, in recent years, in the viewpoint of the environmental problem, there has been required a technique of eliminating the vapor released to the atmospheric air upon opening the cap of the fuel tank for refueling, that is, the so-called puff loss technique. Accordingly, the internal pressure must be equal to or less than the atmospheric pressure upon opening the cap.
An evaporative fuel processing system for satisfying the above requirement has been already proposed, in Japanese Utility Model Application No. hei 3-3883 (1991) by the present applicant.
FIG. 12 shows the above evaporative fuel processing system. A two-way valve 02 and a vent solenoid valve 03 are interposed in parallel to each other in a vent passage 01 for communicating the upper space of an fuel tank T to a canister C, and further, a negative pressure preventive valve 04 is interposed in series to the vent solenoid valve 03. The canister C is communicated to an intake system of an internal combustion engine E by means of a purge passage 05.
In stoppage of the internal combustion engine E, the vent solenoid valve 03 is closed. When the evaporative fuel is generated in the fuel tank T by an increase in the outside air temperature and the like, and the tank internal pressure is increased to be a predetermined value or more, one valve of the two-way valve 02 is opened, and the evaporative fuel in the fuel tank T is introduced to the canister C to be absorbed therein. Thus, it is possible to suppress an excessive rise in the internal pressure of the fuel tank T.
On the contrary, when the internal pressure in the fuel tank T is reduced to be a predetermined value or less by a drop in the outside air temperature and the like, the other valve of the two-way valve is opened, so that the fuel tank T is communicated to the canister C, which makes it possible to avoid an excessive reduction in the pressure within the fuel tank T.
During operation of the internal combustion engine E, the vent solenoid valve 03 is opened, and the upper space in the fuel tank T is communicated to the canister C and the intake system of the internal combustion engine E irrespective of the above two-way valve, which makes it possible to keep the internal pressure in the fuel tank T at the atmospheric pressure or less.
The canister C has an extending ventilation tube 06 at the lower end. Accordingly, in opening the vent solenoid valve 03, the pressure in the fuel tank T can be kept approximately at the atmospheric pressure. However, when there occurs a blocking in the ventilation tube 06 by any cause, the negative pressure in the intake system of the internal combustion engine E is transmitted up to the inside of the fuel tank T through the purge passage 05, which cause a problem of excessively reducing the tank internal pressure. In such a case, the above phenomenon is prevented by closing the negative pressure preventive valve 04.
In the evaporative fuel processing system as shown in FIG. 12, however, in the state that the vent solenoid valve 03 is opened, for example, in operation of the internal combustion engine E, the canister C is forcibly communicated to the fuel tank T, so that the variation in the negative pressure in the canister C generated by a change in the purge amount due to the intake negative pressure of the internal combustion engine E is transmitted to the inside of the fuel tank T as it is, which exerts an adverse effect on the tank strength and the durability. Also, the rapid drop in the internal pressure of the fuel tank has also a problem of causing the vapor lock. Further, the purge amount of the evaporative fuel to the canister C is increased, which promotes the deterioration of the canister C, and the concentration of the air-fuel mixture purged to the internal combustion engine E is largely varied, which makes it difficult to accurately control the air-fuel ratio, thereby causing a problem of deteriorating the purification of the exhaust gas and driveability.
Still further, there is the following disadvantage. Namely, during normal refueling, the upper space of the inside of the fuel tank is closed to obstruct the refueling from exceeding the full-tank fuel level. However, when refueling is performed in a state that the engine is operated, or in the early stage even if after stopping the engine, since the vent solenoid valve 03 is opened and the fuel tank is communicated to the canister, the closing state of the fuel tank cannot be secured, which brings the state of enabling further refueling.
Thus, there is a problem of further continuing refueling, that is, bringing the so-called excessive refueling state. This causes such inconveniences that, in running of an automobile, the fuel in the fuel tank is blown-off from the filler port by the expansion of the fuel or the oscillation of the fuel surface due to the vibration, turning and the like of the vehicle, or the fuel is allowed to flow into the vent passage and to permeate into the canister, resulting in the deterioration of the activated charcoal and the leakage of the fuel from the canister.